Dragging international fast food vendors McDonald's into the row over whether New Zealand is "100 per cent Pure" seems an odd way of trying to damp down the controversy.

Prime Minister John Key says none of the overseas markets targeted by Tourism New Zealand's long-running campaign believe our waterways are 100 per cent pollution-free, any more than they expect to be lovin' McDonald's every time they eat it.

Lovin' every mouthful of McDonald's is marketing hype, says Mr Key, and "it's the same thing with 100 per cent Pure. It's got to be taken with a pinch of salt."

Just as well he didn't add, a pinch of salt, washed down with a glass of 100 per cent fresh New Zealand stream water. At least with a burger, even if you're not lovin' every mouthful, you're not going to end up hugging the toilet bowl for the next 24 hours. But the chances are - I won't say 100 per cent - that if you'd added a glass of water from the stream nearest to your fast food outlet, a bout of vomiting and diarrhoea would be a distinct possibility.

Instead of conceding the campaign is based on a lie and should be quickly retired, the Prime Minister has borrowed the old Muldoonist tactic of attacking the messenger, claiming "one or two academics ... might have been factually incorrect". He claims "the vast bulk of New Zealand waterways are safe to swim in" and that the academic critics "look at the worst ones".

With the world's entertainment media gathered in Wellington for the launch of the first Hobbit movie, he should put his water glass where his mouth is and invite the newshounds to a photo opportunity alongside the pristine Hutt River or some other local stream. It's the stunt former Auckland mayor Sir Dove-Myer Robinson pulled many years ago, at the launch of the Mangere sewage treatment plant to show how pure was the end result being pumped into the Manukau Harbour.

The truth is, Robbie faked it, though he had much less reason to do so than should Mr Key. The Wellington Regional Council's survey of 55 local river sites records 15 as poor and 14 as fair, and that includes the relatively pristine upstream catchments still dominated by unmodified indigenous forest cover.

Or he could take the overseas journalists to the Waikato, home of Hobbiton. The local regional council takes regular samples of rivers, lakes and inland beaches there too. The findings are dire as a graph on the council's website highlights. In lowland Waikato, around 90 per cent of samples were unsatisfactory for contact recreation. The west coast rivers are above 80 per cent off limits, those draining the Hauraki Plains, just under. Around 60 per cent of tests from Coromandel rivers are unsatisfactory.

The website explains the problem: "If water quality is poor then recreational activities such as swimming, kayaking and water-skiing can be unsafe. Sometimes 'bad bugs' from human and animal faeces, micro-organisms such as protozoans, bacteria and viruses can get into waterways. These 'bad bugs' can cause illness when people are exposed to them. Other contaminants such as fine silts and clays reduce the clarity of the water, making it difficult to see and avoid submerged hazards like snags."

The recent State of the Gulf report, recording the continuing decline in the health of the Hauraki Gulf, in part due to the silt and nitrates and other pollutants flowing into the sea from the Waikato, highlights another aspect of environmental decline. A similar picture emerges on any regional council website. Likewise the Ministry for the Environment, which reports more than 50 per cent of freshwater beaches as poor or very poor for recreational use.

Writing on the Crown-owned water and atmospheric research consultancy Niwa's website, Dr Fiona Profitt says: "Despite a comprehensive clean-up of dirty 'point-source' discharges in the 1990s, water quality in many of our lakes and rivers is still declining. The cause this time is 'diffuse-source' pollution associated with intensive land use, particularly pastoral farming."

Monitoring between 1989 and 2009 shows "water quality is appreciably worse at several hundred sites in lowland rivers monitored by regional councils".

Of 134 lakes monitored, 56 per cent are "eutrophic" or worse. This means they suffer from nutrient enrichment that promotes frequent algal blooms, including blooms of toxic cyanobacteria, a type of algae that has plagued central North Island and some South Island lakes over the past decade.

None of this is new, as is nothing in the recent New York Times article, which quotes Massey University environmental scientist Mike Joy saying "there are almost two worlds in New Zealand ... the picture-postcard world, and then there is the reality".

What's embarrassed Mr Key and the tourism industry, is that after romancing the world since 1999 with this fantastical "100 per cent Pure" hype, someone has called their bluff on the eve of The Hobbit launch. Instead of beating on the messenger, the Government should be turning the adman's slogan into a policy goal.

Forcing farmers and industry to stop using waterways as private sewers would be a good first step.

Brian Rudman's first news story was for Auckland University student paper Outspoke, exposing an SIS spy on campus during the heady days of the Vietnam War. It resulted in a Commission of Inquiry and an award for student journalist of the year. A stint editing the Labour Party's start-up Auckland newspaper NZ Statesman followed. Rudman decided journalism was the career for him, but the NZ Herald and Auckland Star thought otherwise when he came job-hunting. After a year on the "hippy trail" overland to London, he spent four years on Fleet St with various British provincial papers. He then joined the Auckland Star, winning the Dulux Journalist of the Year award for coverage of the 1976 Dawn Raids against Polynesian overstayers. He has also worked on the NZ Listener, Auckland Sun, and since 1996, for the NZ Herald as feature writer and columnist. He has a BA in History and Politics.